Sengkang and Punggol rock simply because they were built with
little restrictions.
Having roadside berths won't really work, because it's not
dedicated to the buses. That's the problem with Kranji MRT.
I propose an alternate solution, which is having the interchange
basically being a straight line of sawtooth berths, aligned next to
the (elevated) MRT station, with a parallel bus park, altogether
forming a loop, with entrances/exits on each end.
Obviously there are no more narrow interchange corridors, so I
can dream on (except Boon Lay on the Temp Int site, which is a real
possibility).
Now, it's important to realise that interchanges are integrated
with the MRT for the sake of an integrated interchange. Singapore
is good in the sense that the interchanges are spread out.
However, with the trend being to build the MRT underground, it
would make sense to continue building the interchanges at ground
level over the station site, ala Toa Payoh, with a compact design,
ala Boon Lay.